Coordinate measuring machines frequently are used for the dimensional inspection of workpieces, such as machine parts. A workpiece is secured to a table, and a measuring probe is secured to a movable ram. In order to measure the position of a point on a workpiece, the probe is brought into contact with the point, and the X, Y, and Z measuring scales of the machine are read. Typically, the probe is attached to a vertically movable Z-rail which is also movable in a horizontal plane. Thus, the probe can be moved in three dimensions for contact with a workpiece which is being measured. Typically, the X, Y and Z axis coordinates are each measured using an associated scale and an encoder with a reticle. Either the encoder or the scale can be mounted directly upon the associated rail, while the other of the encoder and scale is mounted to an adjacent support.
Many prior art coordinate measuring machines are designed to be used in controlled, constant temperature environments, so that once calibrated at that temperature, temperature induced errors are not a concern. However, it is not always possible to use these machines in controlled environment conditions, since their use is severely restricted. Where coordinate measuring machines are designed to be used in non-controlled environments with differing temperatures, if the machine is calibrated for use in one temperature environment, use thereof in a different temperature environment may induce uncontrolled errors into the measurement. Errors in the Z-corrdinate readings are most common. The significance of the errors is proportional to the change in temperature. These errors in Z-coordinate readings result from the different materials which may be used to fabricate each of the work support, the probe, and the Z-rail support. Because of the different materials, each of these components may have a different coefficient of thermal expansion, so that a temperature variation can produce differing amounts of expansion in each of these components. As a consequence, such temperature vairations cause a shift to occur in the Z-coordinate reading when the probe is in contact with the work support (i.e., the origin), a phenomenon known as "zero shift." Such a shift in the origin induces corresponding errors into all Z-coordinate measurement when only one point on a workpiece, or one end of a distance to be measured, is determined at any one time. Where an encoder and scale are used, this shift is produced by movement of the scale and encoder with respect to one another.
Correcting for "zero shift" is very difficult especially where the temperature continually changes. The origin of the part must be continually reestablished for each temperature change either mechanically or automatically by a microprocessor controlled system. However, these solutions could result in unnecessarily large delays, and may not readily permit compensation for changes in temperature which occur during the actual measuring process.
It is desirable to provide a coordinate measuring machine which can be used at any temperature within a range of normal temperature environments without requiring continual reestablishment of the origin. It is also desirable to provide a coordinate measuring machine which automatically compensates itself for any temperature variations which occur during a measurement process. It is also desirable to provide a coordinate measuring machine which provides Z-coordinate measurements which are accurate to a high degree of certainty, regardless of the ambient temperature.
It is a general object of the present invention to provide a coordinate measuring machine which can provide highly accurate readings, regardless of the ambient temperature.
It is another object of the present invention to provide a coordinate measuring machine which can provide highly accurate Z-coordinate measurements regardless of the ambient temperature.
It is a further object of the present invention to provide a coordinate measuring machine which automatically compensates for changes in the ambient temperature.
It is yet another further object of the present invention to provide apparatus to compensate for temperature induced shifts in the reference surface of a coordinate measuring machine with respect to the measurement system.
It is a further specific object of the present invention to provide apparatus for offsetting the results of differences in the coefficients of thermal expansion of different parts of a coordinate measuring machine upon which an encoder and scale are mounted for Z-coordinate measurements.